Blown fuse indicator for high-voltage fuses

ABSTRACT

A blown fuse indicator built into a hex-screw and provided with two serially related fusible wire elements of which one is under stress and arranged in a chamber of insulating material forming a void, and the other is wound helically and arranged to be submersed in a body of pulverulent arc-quenching filler.

United States Patent Belcher [111 3,832,665 [451 Aug. 27, 1974 [5 1 BLOWN FUSE INDICATOR FOR HIGH-VOLTAGE FUSES [75] Inventor: Richard A. Belcher, Hampton Falls,

N.H. I

[73] Assignee: The Chase-Shawmut Company, Newburyport, Mass.

2,648,739 8/1953 Lebens, Jr. 337/244 3,601,739 8/1971 Blewitt 337/244 3,621,433 ll/l97l Belcher 337/244 3,678,430 7/1972 Gala 337/244 Primary Examiner-Harold Broome Attorney, Agent, or Firm-Erwin Salzer 5 7 ABSTRACT A blown fuse indicator built into a hex-screw and provided with two serially related fusible wire elements of which one is under stress and arranged in a chamber of insulating material forming a void, and the other is wound helically and arranged to be submersed in a body of pulverulent arc-quenching filler.

5 Claims, 2 Drawing Figures BLOWN FUSE INDICATOR FOR HIGH-VOLTAGE FUSES BACKGROUND OF THE INVENTION This invention relates to an improvement of the fuses and of the blown fuse indicator thereof disclosed and claimed in US. Pat. No. 3,621,433 to Richard A. Belcher, Nov. 16, 1971 for ELECTRIC CARTRIDGE FUSE HAVING PLUG TERMINALS.

It is one object of this invention to provide a blown fuse indicator having advantages in addition to those of the blown fuse indicator disclosed inthe above patent. The blown fuse indicator according to the present invention has two serially connected fusible elements of which one is under stress and is well insulated thermally and not subject to the danger of being accidentally short-circuited, and the other is virtually under no stress and can be wound helically, as desirable, wher everthe circuit voltage is relatively high and is, in essence, a means for voltage distribution.

Another object of this invention is to provide a blown fuse indicator which lends itself particularly well to be applied in connection with fuses dispensing with a supporting mandrel for the helically wound currentcarrying fusible elements, which fuses are manufactured as taught in the copending patent application of Erwin Salzer, filed 7/9/71 Ser. No. 161,089 for METHOD OF ASSEMBLING ELECTRIC HIGH- VOLTAGE FUSES AND SUBASSEMBLY THERE- FOR.

SUMMARY OF THE INVENTION A blown indicator built into a hex screw screwed into one of a pair of plug terminals of a fuse for elevated circuit voltagesThe hex screw supports on the axially inner end of the shank thereof a closed coaxially arranged tubular member of electric insulating material. A fusible restraining wire for the spring-biased indicator pin of the blown fuse indicator is arranged inside said tubular member and affixed with one end thereof to said tubular member. An additional fusible wire is arranged in series with said restraining wire. One end of said additional fusible wire is affixed to said tubular member and the other end of said additional fusible wire is affixed to the other of said pair of plug terminals of the fuse.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows in vertical section a blown fuse indicator embodying the present invention; and

FIG. 2 shows on a smaller scale'than FIG. 1 a topplan view of a fuse and blown fuse indicator embodying the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT Reference numeral 1 has been applied to indicate a pair of cylindrical plug terminals which are arranged in coaxial relation and plug the axially outer ends of a tubular casing of electric insulating material, e.g. melamine glass cloth. FIG. 1 shows but portions of plug terminals 1, other portions thereof being broken away. Plug terminals 1 are affixed to casing 2 by steel pins 30 projecting transversely through casing 2 into plug terminals l. Casing 2 is filled with a pulverulent arcquenching filler, and plug terminals 1 are conductively interconnected by current-carrying fusible elements, preferably in the shape of helical windings. The current-carrying fusible elements are not shown in FIGS. 1 and 2. These elements may be arranged and supported as shown in the above referred-to US. Pat. No. 3,621,433.

As shown in FIG. 1, the first or upper plug terminal 1 has a centrally located internally screw-threaded axially extending bore 3 of relatively large diameter. A hex screw 4 is inserted into bore 3 and closes the latter. Hex screw 4 has an axially extending bore 5 of relatively small diameter. Bore 5 houses a spring-biased blown fuse indicator which includes the indicator pin 6 and the helical pin-biasing spring 7. Reference numeral 8 has been applied to indicate a tubular member of electric insulating material, e.g. fiber, arranged in coaxial relation to, and affixed to, the axially inner end of the shank of hex screw 4. The axially inner end of tubular member 8 forms a shoulder 9 and cover 10 resting against shoulder 9 closes member 8, thus precluding the pulverulent arc-quenching filler outside of member 8 from entering into the latter. Reference numeral 11 has been applied to indicate a fusible restraining wire for indicator pin 6. Indicator pin 6 is provided with a narrow bore 12 forming a shoulder adjacent the axially inner end of pin 6. Restraining wire 11 is provided with a knot resting against the aforementioned shoulder and is thus affixed to indicator pin 6. The other end of restraining wire 11 is affixed at 14 to tubular insulating member 8. Since the upper end of helical spring 7 rests upon a collar formed by pin 6, and since restraining wire 11 is affixed to pin 6, restraining wire 11 as long as intact is stressed by spring 7. Reference numeral 15 has been applied to indicate an additional fusible element in wire-form connected in series with restraining wire 11. Fusible element 15 is preferably in the form of a substantially helical winding of wire having a high resistivity. Such a winding forms a weak spring capable of exerting but a small fraction of the force which helical compression spring 7 is capable of exerting. The upper end of fusible element 15 is affixed to the axially inner end of insulating member 8 and the lower end of fusible wire 15 is affixed to the second or lower plug terminal 1 by means of clamping screw 16, and thus conductively connected to second or lower plug terminal l. The bore 5 in hex screw 4 includes portions of different diameter forming a shoulder 17. Reference numeral 18 has been applied to indicate a multi-layer disk of metal and an elastomer, e.g. a round plate of silicone rubber may be sandwiched between a pair of metal plates to form the aforementioned multi-layer disk. Disk 18 rests against shoulder 17 and spring 7 rests against one side of disk 18. Tubular member 8 abuts against the other side of disk 18. Disk 18 is provided with a narrow bore 19', and restraining wire 11 is threaded through the latter. The axially outer end of insulating member 8 is provided with an annular groove 19 receiving a radially inwardly crimped projection 20 of hex screw 4, thus firmly affixing part 8 to part 4. The axially inner end of insulating member 8 is provided with an annular groove 21 receiving several turns of fusible element 15, and thus affixing its upper end firmly to part 8. The second or lower plug terminal 1 has a centrally located internally screw-threaded axially extending bore 3' of relatively large diameter which is closed by an additional hex screw 4. The lower plug terminal 1 forms a passageway 22 allowing to connect the lower end of fusible element 15 to the axially outer end surface of lower plug terminal 1. Hex

screws 4 and 4' are also used for affixing and electrically connecting a pair of conductive straps 23 to the axially outer end surfaces of the pair of plug terminals 1. Pins 24 project through the heads of hex screws 4 and 4 and through conductive straps 23 into plug terminals 1 and thus preclude any unintentional rotation or displacement of hex screws 4,4 relative to plug terminals 1.

It will be apparent from the above that fuse indicators embodying this invention are bench-assembled selfcontained sub-assemblies. In order to combine such a sub-assembly with a fuse structure first hex screw 4 and all the parts supported by it are joined with the upper plug terminal 1 by screwing said screw 4 into bore 3. Thereafter the lower end of fusible wire 15 is affixed by means of screw 16 to the axially outer end surface of lower plug terminal 1. Thereafter the casing 2 of the fuse is filled with a pulverulent arc-quenching filler, e. g. quartz sand, through the large bore 3 in lower terminal plug 1, i.e. the structure shown in FIG. 1 is inverted for the purpose of filling the same with a pulverulent arcquenching filler. Thereafter bore 3 is closed by hex screw 4. I

Before insertion of hex screw 4 into bore 3 spring 7 must be loaded and indicator pin 6 pulledaxially inwardly into bore 5. This is effected by exerting an axial pull upon restraining wire 11, and then affixing its lower end at 14 to part 8.

When assembling a fuse in accordance with the teachings in the above referred-to co-pending patent application of Erwin Salzer the upper plug terminal 1 may be provided with passageways for insertion of temporary rod supports for the fusible element or elements, and the lower plug terminal 1 may be provided with blind holes for receiving the ends of said temporary rod supports. The casing of the fuse is then filled with pulverulent arc-quenching filler, as described above, while the fuse is in an inverted position and the aforementioned temporary rod supports for the fusible element or elements are withdrawn from the casing of the fuse in downward direction while the fuse is in its inverted position. v

In FIG. 1 the indicator pin 6 is shown in its outward or indicating position. Indicator pin 6 assumes that position either when restraining wire 11 has formed a break inside of member 8, (where the temperature is particularly high, tending to form a break at this point) or before spring 7 has been loaded by exerting an axial pull upon restraining wire 11. It will be thus apparent that pin 6 will never assume the position shown to FIG. 1 when the fuse is fully assembled and restraining wire 11 intact. FIG. 1 is, therefore, but an imaginary position of the parts of the blown fuse indicator which has been selected to better show the constituent parts of the blown fuse indicator. The blown fuse indicator structure might not be shown as clearly if spring 7 where shown as being loaded, and pin 6 in its axially inner position inside of bore in hex screw 4.

The multi-layer disk 18 is also an effective means for precluding hot products of arcing formed incident to blowing of the fuse to leak to the outside of the same.

Since all the stress is imposed upon straight wire 11 and virtually none upon helical wire 15, spring 7 can be very strong and pin.6 thus subject, when released, to a considerable force of many lbs. Thus pin 6 may not only be used to visually indicate blowing of a fuse but also as a striker pin. If applied as'a striker pin, pin 6 is capable of unlatching latches against considerable latch friction.

It will be understood that the resistance or serially connected wires ll, 15 by far exceed the resistance of the fusible current-carrying elements normally conductively interconnecting the pair of plug terminals 1. Wires 11 and 15 begin to carry a significant current only when the current path of low resistance formed by the fusible current-carrying elements has been destroyed by fusion of the latter. Then a break is relatively quickly formed in the current path formed by wires 11,15 at a point situated inside of tubular insulating member 8. The cover 10 of that member forms a passage for restraining wire 11 which is so narrow as to preclude entry of pulverulent arc-quenching filler from the space outside member 8 into the void formed by member 8.

I claim as my invention:

1. A blown fuse indicator for high-voltage fuses having a pair of plug terminals including a. a first plug terminal having a centrally located internally screw-threaded axially extending bore of relatively large diameter;

b. a hex screw inserted into and closing said bore, said hex screw having an axially extending bore of relatively small diameter;

c. a spring-biased blown fuse indicator arranged in said bore of relatively small diameter and including an indicator pin and a pin-biasing helical spring;

d. a closed tubular member of electric insulating material arranged in coaxial relation to, and affixed to, the axially inner end of the shank of said hex screw;

e. a fusible restraining wire for said indicator pin having one end affixed to said indicator pin and another end affixed to the axially inner end of said tubular member and put under stress by said pinbiasing spring;

f.- a second plug terminal; and

g. an additional fusible element in wire-form connected in series with said restraining wire, one end of said additional fusible element being affixed to said axiallyinner end of said tubular member and the other end of said additional fusible element being affixed to said second plug terminal.

2. A blown fuse indicator as specified in claim 1 wherein said bore of said hex screw has portions of different diameter forming a shoulder therebetween, said shoulder positions a multi-layer disk of metal and of an elastomer, said pin-biasing spring rests against one side of said disk and the axially outer end of said tubular member abuts against the other side of said disk, and wherein said disk has a narrow bore in the center thereof and said restraining wire is threaded through said narrow bore.

3. A blown fuse indicator as specified in claim 2 wherein said axially outer end of said tubular member is provided with an annular groove receiving a radially inwardly crimped projection of said axially inner end of said shank of said hex screw, and-wherein said axially inner end of said tubular member is provided with an annular groove receiving several turns of said additional fusible element.

4. A blown fuse indicator as specified in claim 1 wherein said additional fusible element is in the form of a substantially helical winding.

3,832,665 6 5. A blown fuse indicator as specified in claim 1 centrally located internally screw-threaded axially exwherein said second plug terminal is provided with a tending bore of relatively large diameter which is wire-clamping screw at the axially outer end surface closed by an additional hex screw. thereof, and wherein said second plug terminal has a 

1. A blown fuse indicator for high-voltage fuses havIng a pair of plug terminals including a. a first plug terminal having a centrally located internally screw-threaded axially extending bore of relatively large diameter; b. a hex screw inserted into and closing said bore, said hex screw having an axially extending bore of relatively small diameter; c. a spring-biased blown fuse indicator arranged in said bore of relatively small diameter and including an indicator pin and a pin-biasing helical spring; d. a closed tubular member of electric insulating material arranged in coaxial relation to, and affixed to, the axially inner end of the shank of said hex screw; e. a fusible restraining wire for said indicator pin having one end affixed to said indicator pin and another end affixed to the axially inner end of said tubular member and put under stress by said pin-biasing spring; f. a second plug terminal; and g. an additional fusible element in wire-form connected in series with said restraining wire, one end of said additional fusible element being affixed to said axially inner end of said tubular member and the other end of said additional fusible element being affixed to said second plug terminal.
 2. A blown fuse indicator as specified in claim 1 wherein said bore of said hex screw has portions of different diameter forming a shoulder therebetween, said shoulder positions a multi-layer disk of metal and of an elastomer, said pin-biasing spring rests against one side of said disk and the axially outer end of said tubular member abuts against the other side of said disk, and wherein said disk has a narrow bore in the center thereof and said restraining wire is threaded through said narrow bore.
 3. A blown fuse indicator as specified in claim 2 wherein said axially outer end of said tubular member is provided with an annular groove receiving a radially inwardly crimped projection of said axially inner end of said shank of said hex screw, and wherein said axially inner end of said tubular member is provided with an annular groove receiving several turns of said additional fusible element.
 4. A blown fuse indicator as specified in claim 1 wherein said additional fusible element is in the form of a substantially helical winding.
 5. A blown fuse indicator as specified in claim 1 wherein said second plug terminal is provided with a wire-clamping screw at the axially outer end surface thereof, and wherein said second plug terminal has a centrally located internally screw-threaded axially extending bore of relatively large diameter which is closed by an additional hex screw. 